Applications for the industrial synthesizing of clathrate hydrates and semi-clathrates (hereafter referred to as “gas hydrates” or “hydrate,” except when differentiation is necessary) include desalination, gas storage, gas transport, and gas separation. Considerable work has been applied to the field of applied physical chemistry of these systems over the past 50 years in order to develop commercial technologies. To our knowledge, none have succeeded in producing a viable innovation for gas separation (although some clathrate hydrate-based processes for transport and desalination on a commercial scale appear close to success). Using gas hydrate systems to separate gases is a recent endeavor that has been mainly focused on extraction of CO2 from combustion exhaust to keep it from emitting into the atmosphere.
In general, clathrate hydrates and semi-clathrates are a class of non-stoichiometric crystalline solids formed from water molecules that are arranged in a series of cages that may contain one or more guest molecules hosted within the cages. For clathrate hydrates, the whole structure is stabilized by dispersion forces between the water “host” molecules and the gas “guests.” Semi-clathrates are very similar to clathrate hydrates except one material (“guest material”) serves “double-duty” in that it both contributes to the cage structure and resides at least partially within the cage network. This special guest can be ionic in nature, with tetrabutylammonium cations being a classic example.
Hydrate formed from two or more species of molecule (e.g., methane, ethane, propane, carbon dioxide, hydrogen sulfide, nitrogen, amongst others) is referred to by several names: compound hydrate, mixed-gas hydrate, mixed guest hydrate, or binary hydrate. Each hydrate-forming species has a relative preference to enter the hydrate-forming reaction from any gas mixture and each hydrate has a range of cage sizes that can accommodate the guests. Tetrabutylammonium cation semi-clathrates differ from clathrate hydrates in this regard in that they only have one, small cage. They are thus more size selective than clathrate hydrates. Controlled formation of compound hydrate can be used to separate gases based on high and low chemical preference for enclathration or by size rejection (“molecule sieving”) in the mixture. Species with a high preference dominate the species in the hydrate while low preference gases are not taken into the hydrate in relation to their percentage of the original mixture and are thus “rejected.” Similarly, gases that are too big to fit in the hydrate cages are rejected; again, this is more critical for semi-clathrates than clathrate hydrates.
The controlled artificial production of hydrates is challenging because the natural rate of hydrate formation and dissociation may need acceleration in order for it to be used as the basis of a fully commercial process. Acceleration of the reaction rate of hydrate processes has focused on the role of a certain class of molecules that act as catalysts for hydrate formation and dissociation. Catalysts have been found to increase the rate of hydrate formation and dissociation reactions by orders of magnitude compared to uncatalyzed systems. See Ganji, et al. (2007) “Effect of different surfactants on methane hydrate formation rate, stability and storage capacity,” Fuel 86, 434-441 (“Ganji 2007). Certain prior art references have focused on the artificial growth aspect of gas hydrate. The use of various additives to increase the growth rate (U.S. Pat. No. 5,434,330, for example) and to promote hydrate growth at lower pressures (U.S. Pat. No. 6,855,852 (discredited by Rovetto, et al. (2006) “Is gas hydrate formation thermodynamically promoted by hydrotrope molecules?,” Fluid Phase Equilbria, 247(1-2), 84-89)), or by adding additional hydrate-forming “helper” gases (U.S. Pat. Nos. 6,602,326 and 6,797,039) have been considered only for the impact on formation rates and not on the total process rate, or throughput. The impact of these accelerative processes on dissociation does not appear to have been investigated in a systematic manner with respect to the complete processing of gas, for separation or for any other purpose. Not only must hydrate formation be accelerated, but also nothing should be done to inhibit any other stage of the process.